The present invention relates to an apparatus and a method for reproducing information from a recording medium by using holography.
At present, by a Blu-ray Disc™ standard using a blue-violet semiconductor laser, optical discs having a storage capacity of about 50 GB can be commercialized for general use. In the future, optical discs are desired to have increased capacities as much as those of HDDs (Hard Disk Drive) of 100 GB to 1 TB.
However, to realize such an ultra-high density by an optical disc, a high-density technique by another new method is required the method unlike high-density techniques of making a laser have a short wavelength and making an objective lens have a high NA (Numerical Aperture).
Amid studies for next generation storage technology are being conducted, attention is being given to a hologram recording technique of recording digital information by using holography.
The hologram recording technique is a technique in which a signal beam having information on page data that is two-dimensionally modulated by a spatial light modulator is superposed on a reference beam inside a recording medium, and refractive index modulation occurs inside the recording medium by an interference fringe pattern that is generated at this point, to record the information on the recording medium.
At the time of reproducing the information, applying the reference beam used at the time of recording to the recording medium causes the hologram recorded in the recording medium to function like a diffraction grating to generate a diffracted beam. The diffracted beam is reproduced as a single beam of light including the recorded signal beam and phase information.
The reproduced signal beam is quickly and two-dimensionally detected by using a photodetector (camera) such as a CMOS and a CCD. In such a manner, the hologram recording technique can record two-dimensional information on an optical recording medium in a single hologram at once, and can reproduce the information, which can overwrite a plurality of pieces of page data at a certain portion in the recording medium, achieving high-capacity and high-speed recording/reproducing of information.
JP-A-2006-172582 (Patent Literature 1) discloses the hologram recording/reproducing technology, for example. This publication describes that “In addition, a mechanical shutter 25 for opening/closing a beam at the time of recording is disposed between an half-wave plate 24 and a polarizing beam splitter 26, and a period of recording time is determined by the timing of opening/closing the mechanical shutter 25.”
In addition, JP-A-2007-4041 (Patent Literature 2) discloses the hologram recording/reproducing technology, for example. This publication describes that “A pattern providing such polarized light that all of the signal light 100 are shut off by a polarizing plate 14 during a period between data recording and data recording is displayed on a spatial modulator (SLM) 13 for performing spatial modulation of the signal light 100. As a result, a hologram recording material 50 is no longer irradiated with the signal light 100 between the recording and the recording, and therefore, the continuation of the useless recording on the hologram recording material 50 can be prevented. Also, the spatial modulator 13 functions as the shutter but the operation is rapid and does not generate the vibrations and therefore, the exact signal can be recorded and the transfer rate can be made higher.”
In addition, JP-A-2004-30726 (Patent Literature 3) is disclosed, for example. This publication describes that “As a method for minimizing spherical aberration, a method or the like can be employed in which a wave surface on a liquid crystal element 20 is adjusted so as to minimize jitter components generated due to spherical aberration while signals recorded in an optical recording medium 10 disc are reproduced.”
In addition, JP-A-11-242827 (Patent Literature 4) describes that “In addition, since a liquid crystal shutter 14 is a liquid crystal shutter configured by a plurality of pixels, any portion of feedback light incident on a detector 11 can be shielded by controlling the individual pixels. As a result, for example, even when an objective lens 6 is shifted and an optical axis thereof deviates, which moves a portion including much of information on a guiding groove edge region 73, a light-shielding position can be moved, accordingly.”